1. Field of the Invention
The present invention relates to formation of a fine pattern of a semiconductor device. Specifically, with regard to a dimensional variation of a resist pattern in a photolithography process, the present invention relates to an evaluation method of dependence on a mask pattern coverage, a correction method for a mask pattern to prevent dependence on a mask pattern coverage, a manufacturing method for a semiconductor device, and a computer program product for evaluating dependence on a mask pattern coverage.
2. Description of the Related Art
In the continuing progress of miniaturization of a semiconductor device, a technique for precisely controlling a line width of a pattern formed on a semiconductor substrate by photolithography is required. In a one-shot exposure area, dimensions of a resist pattern provided by transferring a mask pattern of a photomask may vary in some cases. A dimensional variation in a resist pattern leads to a dimensional variation in a circuit pattern of a semiconductor device, to negatively affect operations of the semiconductor device. Therefore, the dimensional variations in the resist pattern needs to be reduced as much as possible.
One of the causes of variation in a dimension of a resist pattern transferred by a mask pattern uniformly provided on a photomask is an optical proximity effect (OPE) which occurs in an exposure process. The OPE conspicuously appears in a pattern in a dimension proximate to a resolution limit of an exposure tool. The OPE can be prevented by optical proximity correction (OPC) which corrects dimensions of a mask pattern by predicting a dimensional variation from the photolithography simulation in view of a pattern arrangement in a proximity area within several μm of a target pattern.
In addition, besides the OPE which occurs in the exposure process, a process proximity effect (PPE) which occurs in a post-exposure baking (PEB) process and a development process, also vary dimensions of a resist pattern. With regard to the PPE, the dimensional variation in the resist pattern can be prevented by modifying each process based on a relationship between dimensional variation of a target pattern and a condition of a pattern surrounding the target pattern, which is experimentally provided by process simulations, or by correcting the dimension of the mask pattern.
Influence of a coverage condition of the mask pattern of the photomask in a relatively wide range surrounding the resist pattern has been a problem with the dimensional variation in a fine resist pattern. The “relatively wide” range refers to a range within about 100 μm to about 1000 μm of the target pattern. The relatively wide range is far wider than the proximity area on which the OPE and the PPE have influence.
For example, in the case of mask patterns of a random access memory (RAM) and a read-only memory (ROM), a peripheral circuit is provided for controlling a memory cell array and a memory cell. Fine patterns, for example, line and space (L/S) patterns with a width ratio of a line to a space of about 1:1, are densely arranged in the memory cell array area where a mask pattern coverage is large. The memory cell array area has a rectangular shape of about 5 mm on a side, for example. In the peripheral circuit area, opaque patterns of the photomask are less than the memory cell array area. Thus, the peripheral circuit area has a smaller local mask pattern coverage of the photomask.
A mask pattern including the memory cell and the peripheral circuit is transferred onto a resist film to measure line widths of the L/S pattern along a diagonal line of the rectangular shaped memory cell array area. With regard to a line width of an L/S resist pattern transferred onto the resist film, the line width may be thinner in an end portion of the memory cell array area than the center portion of the memory cell array area, regardless of the constant line width of the L/S pattern on the photomask. A boundary in which a dimension in the resist pattern may vary in reference to the center portion of the memory cell array is located about 100 μm to about 1000 μm away from the end portion of the memory cell array area, although depending on an exposure tool. The dimensional variation in the resist pattern depending on the local mask pattern coverage of the photomask has been an increasingly large problem, particularly as the circuit pattern to be formed becomes finer.
Reasons for the dependence of the dimensional variation in the resist pattern on the mask pattern coverage include a local flare (also refered to as a “mid range flare”) which occurs in an optical system of the exposure tool, transpiration and re-deposition of an acid which occurs from the resist film in the post-exposure baking (PEB) process, as well as a development microloading effect of a resist image. The “flare” refers to a noise in a projection light, which does not contribute to image formation to deteriorate contrast of an aerial image. The “local flare” refers to a flare which is distributed depending on a shape of a mask pattern, particularly on a local mask pattern coverage of a photomask. Generally, the local flare is smaller in the vicinity of an area having a larger mask pattern coverage, whereas the local flare is larger in the vicinity of an area having a smaller mask pattern coverage. In addition, the “development microloading effect” refers to a phenomenon in which development rates vary depending on a pattern density.
A method for correcting of a line width of a mask pattern having a plurality of transparent areas and opaque areas has been proposed based on an amount of local flare of a lens of an exposure tool, and a ratio of sums of the transparent areas and the opaque areas in the photomask (see Japanese Patent Laid-open Official Gazette No. 2003-100624). The amount of the local flare of the lens is quantified by comparing line widths of resist patterns transferred from the photomask. However, in the case of the method as disclosed in the patent literature, periods of the transparent areas and the opaque areas are not taken into consideration. A large error can occur in a line width thus corrected when the periods of the transparent areas and the opaque areas is approximately equal to a wavelength of exposure light in terms of a pattern dimension transferred on the semiconductor substrate.
Dimensional variations in resist patterns caused by a resist material and a development process are evaluated, for example, by use of a plurality of photomasks, each of which has a different mask pattern coverage with regard to a peripheral area of a mask pattern, such as a L/S pattern. However, a dimension error distribution in a mask pattern may occur due to dependence on a mask pattern coverage inherent in an electron beam lithography system and the like to be used to fabricate the photomask. For this reason, a dimension error distribution in a mask pattern of an inspection photomask is measured prior to correcting a measured dimension value of a transferred resist pattern. The dimension error in the mask pattern is required to be corrected for each of the photomasks having the diffrent mask pattern coverages. Therefore, the evaluation of the dimensional variations in the resist pattern may be complicated, and may require a longer time.